1. Field of the Invention
The invention described herein is directed to prosthetic valves, such as mechanical heart valves adapted for surgical implantation to a patient. More specifically, the invention is operable to mitigate audible noise, physical discomfort, and mechanical stress on both the replacement valve and patient tissues that is caused from hydraulic shock in the fluid when the valve through which the fluid flows undergoes a transition from an open position to a closed position.
2. Brief Description of the Prior Art
The surgical replacement of natural heart valves with durable, artificial valves in living patients has become commonplace due to the reliability of such valves achieved in the past several decades. Among the prosthetic replacement valve options are “tissue valves”, which are valves taken from deceased donors or even animals, and “mechanical valves”, which replace the soft tissue valves with those of various biocompatible manufacturing materials. Both of these options suffer from various problems and deficiencies.
Mechanical valves, due to the presence of fixed hard edges and other biological incompatibility factors, are prone to the formation of potentially damaging blood clots in the recipient patient. Thus, anti-coagulation therapy is generally required as an adjunct to mechanical valve implants. Tissue valves do not require such anti-coagulation treatment, but tend to stiffen and calcify over a relatively short lifetime (such as after ten years) and then require replacement. Despite the requirement of sustained medical treatment with anti-coagulation drugs, mechanical valves are often preferable over tissue valves in patients expected to live postoperatively longer than ten or fifteen years, because implanting a prosthetic valve may reduce the likelihood of additional surgery to replace the valve.
Considerable design effort has been directed towards moderating the drawbacks of mechanical prosthetic valves. Of these drawbacks, mechanical valve noise has been brought forward as a prominent complaint by many mechanical valve recipients. Tissue valves are quiet due to certain natural properties inherent to their leaflet and substrate elasticity and because of their quasi-optimal evolutionary shape. Mechanical valves by contrast, due to limitations of manufacturing processes and materials, can be quite noisy. Noise problems are compounded in the presence of a graft, such as an aortic root replacement, that removes the acoustic and shock absorptive properties normally afforded by the surrounding natural tissue structures. Unfortunately, it is common for both the ascending aorta and aortic valve to be replaced simultaneously in the same operation, such as for patients with an aortic aneurysm.
A typical contemporary artificial heart valve includes a fixed annulus in which are suspended one or more gating flaps or “leaflets”. This design has replaced early ball-and-cage designs, which have since been outmoded. The leaflets are hinged so as to open by blood flow during systole and to close and seal against the surrounding orifice during diastole, when blood flow begins to reverse. Generally, mechanical valves are composed of metals, carbon composites, pyrolytic carbon, or various hybrid combinations that are fairly stiff and durable, but the closing of these valves introduces noise that is both heard and felt by mechanical valve recipient patients.
There are two distinct components to the noise generated by the valve when it is closed. A first component arises from the impact of the leaflets against the orifice. In patients that have received a mechanical valve, this is typically experienced as an audible “click” or metallic “tink” sound occurring at each heartbeat. Since these valves close under considerable pressure, the impact forces can be substantial and the resulting sound may be quite loud.
A second component of valve noise arises from the phenomena of hydraulic shock. Whenever a fluid flow is suddenly interrupted, it creates a shock wave that propagates back through the system by way of the fluid medium and resonates until it is damped by the natural properties of the system. The force of this shock can be quite strong. In plumbing, a similar phenomenon is referred to as “water hammer” and the forces generated are known by plumbers to be potentially damaging in magnitude.
The noise components of mechanical heart valves are well documented by patient recipients thereof. These patients, and even their families, complain of a ticking or similarly unnatural sound emanating from the patient and over time, adapt to accommodate them. The ticking sound of a valve leaflet closing is clearly audible to third parties, definitely mechanical in tone, and present in nearly all patients, regardless of body fat or other factors.
The noise attributed to hydraulic shock is not clearly audible to an external party without deliberate observation, such as through a stethoscope or by close contact. This noise component has a much duller and “natural” tone and its amplitude varies depending upon the recipient's breathing, body position, and body fat. Previously, the focus of physicians and other specialists has been on addressing the most obvious issues first. Thus, there are numerous devices that concern themselves with moderating the ticking heart valve noise of the valve leaflet striking the sides of the orifice. Such designs of the prior art have generally focused on slowing or modifying the final closing stages of the leaflets so as to lessen the impact noise. The second, shock component of the noise, however, has been largely ignored by heart valve designers.
Nonetheless, shock noise is a persistent complaint in patients. Due to sound conduction through tissue and bone, the shock noise is much more audible to the patient than to an external party. Additionally, the hydraulic shock is sometimes extremely strong and has very low frequency components that may present an uncomfortable physical “thumping” sensation in the chest wall.
Psychoacoustic stress notwithstanding, the hydraulic shock involved in these mechanical replacement valves present physical forces that may very well shorten the life of the valve and may cumulatively damage major blood vessels or other body tissue. The hydraulic shock is a particular problem in aortic valve replacements in that the aortic valve is situated between the final pumping chamber of the heart and the aorta. The blood is pushed through the aortic valve to the entire body and thus, the pressures are highest and the shock greatest at that point.
In the natural cardiovascular system, soft tissue in arterial walls in certain cavity structures known as “sinuses” act to absorb and dampen hydraulic shock. The mechanical valve, however, generates a much more abrupt interruption of flow than a tissue valve due to the stiffness of the components composing the valve. Natural structures may be inadequate for absorbing the shock. Furthermore, when a synthetic arterial graft is introduced with the mechanical valve, as is common in aortic aneurysm repair, the combined system loses many of the natural dampening features and the problem is significantly worsened.
Referring now to FIGS. 1A and 1B, there is shown a valve and graft combination of the prior art having artificial sinuses incorporated in section of a graft downstream of the aortic valve. The prosthetic graft section 100 is adapted with the flexible elastic wall 130 that expands under high pressure and contracts with lower pressure. In FIG. 1A, the systolic stage of the heart beat cycle pushes blood through the valve 110 and thereby causing leaflets 120a, 120b to open by the force of the blood flow. At the outset of this stage, the elastic walls 130 are in a relatively contracted state and begin to stretch as they are filled with high-pressure blood from the heart. During the diastolic stage of the heart beat cycle, as shown in FIG. 1B, the blood flow begins to reverse at the valve and thereby forces the leaflets to close against the orifice 115. The pressure shock wave introduced to the fluid by the closing of the valve is transmitted in part to the elastic walls 130, which then expand an additional amount to absorb the shock energy. The effect of the device is two-fold. First, it dampens the pulses of each heart beat to a more sustained flow, as would the flexible walls of a natural aorta. Secondly, the elasticity dampens shock waves that would otherwise resonate through the system had a stiff tube been implemented for the artificial aorta.
The design illustrated in FIGS. 1A-1B is not optimized to effectively dampen hydraulic shock. The elastic section 130 stretches under pressure during the ejection of blood from the heart. Then, as the blood flow reverses, the elastic section 130 contracts and contributes to a force that causes the leaflets 120a, 120b to close against the orifice 115 slightly harder and faster than would occur had the elastic walls not been introduced. Furthermore, the shockwave caused by the mechanical transition of state of the valve will be only partially damped by the elastic walls. Additional shockwaves reflected through the system and back to the valve will also be only partially damped by the semi-relaxed sinus walls, i.e., the sinuses will not be fully relaxed when the shockwave arrives. Thus, dampening will not be optimal. Moreover, the elastic expansion occurs perpendicular to blood flow, i.e., in a radial direction, and is therefore not well suited to absorbing shock energy from a compression wave moving longitudinally through the system.
An additional problem arises with artificial sinuses in that they are susceptible to scarring. It is common in aortic grafts that accumulation of a centimeter or more of fibrous scar tissue forms as the body reacts to the presence of the artificial material. This fibrous tissue is much less flexible than the natural arterial walls and less flexible than the artificial sinus cavity walls. Over time, the scar tissue will tend to stiffen the elastic walls, which then precludes expansion and contraction. Should the walls stiffen to a point where they are maintained in a position that causes blood to eddy, the resulting hemodynamics may lead to increased clot formation in the region.
Additionally, the use of an artificial sinus only applies to patients that are receiving an aortic graft in addition to a replacement valve. An expandable aortic graft is not introduced into a patient who requires only valve replacement. Thus, the need has been felt for a prosthetic mechanical valve which mitigates potentially damaging shock forces in any patient that receives such an artificial valve.